Belgse could not have crossed the fosse without subjecting them-
selves to a double artillery fire. Authors of later date, Hyginus
probably in the fourth century and Vegetius certainly of that
period, speak of such projectile engines as established necessities
equally for a fortification and for a campaign. The very last
sentence of the extant work of Hyginus1 says that the vallum
(of a camp, that is) ought to be best furnished with artillery at
its weakest parts, where disadvantage of site was inevitable.
It is not difficult to believe that it was because the expedient
allowed a double frontage that the same passage appears to
recommend the placing of the engines in strong force in the
rounded corners of the camp.

Relying thus greatly on projectile engines, the Romans never
went on a campaign without a .large supply. It was one of the
duties of the prefect of the camp2 to see to them—the onager,
the ballista, and the other kinds of tormenta. Vegetius makes it
clear that in the military system these engines played a large
and vital part. He turns again and again to speak of them with
admiring enthusiasm. " Stones thrown by the onager," he says,3
" not only kill men and horses, they break the enemy's engines
as well." He tells how the onager discharges stones varying in
size with the thickness of its horse-hair spring—" throws them,"
says he, "like lightning." And he declares4 that there is no
kind of engine more powerful than the ballista and the onager.
Elsewhere he reiterates5 this—"Indeed, the ballista and the
onager, if they are skilfully handled, excel all other means of
defence; from them neither valour nor fortifications avail as a
defence. For, like lightning they strike, and what they strike